Electronic circuitry provides complex functionality that is proving ever more useful. Some circuits even operate in one of multiple operation modes, typically a normal operation mode and a single alternative operation mode. For instance, a circuit may operate in configuration mode when not in a normal mode of operation. Other examples of alternative operation modes include, for example, testing mode, calibration mode, and the like.
There are a variety of techniques for transitioning a given circuit from one operation mode to another. Many involve signaling the circuit using voltages that are not typical for normal operation of the circuit. For instance, the transition may be initiated by modulating the power supply outside of its normal range. For circuits that are flexible enough to operate normally with a wide range of supply voltages, it may be quite difficult or impractical to modulate the power supply to be outside of its normal range. Another mechanism is to overdrive the voltage on an input/output (I/O) pin of the circuit. However, for circuits that have a low drive resistance, this may cause excessive current to pass through the circuit.